On Tuesday night, Jan. 5-6, there's a fair chance that the northern lights will put on a little show for skywatchers in the northern U.S., Canada and northern Europe. NOAA space weather forecasts predict a minor (G1) geomagnetic storm from around 9 p.m. to midnight Central Time. Minor storms often begin with the appearance of a faint, pale-green arc several degrees thick about a fist high across the northern sky. To see it, you'll need an open view in that direction and time to allow your eyes to adjust to the darkness, usually about 10 minutes.

Parallel rays stretch upward from a low arc during an auroral display near Duluth, Minnesota, several years ago. (Bob King)
Parallel rays stretch upward from a low arc during an auroral display near Duluth, Minnesota, several years ago. (Bob King)

Often a second arc will appear above or below the first. If the storm heats up, vertical columns of light called rays appear above the arc and move slowly westward. The movement reminds me of a shiver of sharks, their fins slicing the waves. More dramatically, the arc can suddenly break up into multiple rays resembling the pleats of a skirt.

A filament, seen at right, appears dark in silhouette against the brighter sun. The extension of the filament off the sun's edge appears as a bright prominence against the blackness of space. (NASA)
A filament, seen at right, appears dark in silhouette against the brighter sun. The extension of the filament off the sun's edge appears as a bright prominence against the blackness of space. (NASA)

Tonight's potential aurora is brought to you by the combined effects of a coronal hole and two ejections of solar plasma called filaments in Earth's direction on Jan. 2. Coronal holes are gaping holes in the sun's magnetic canopy that allow particles to stream directly from the sun into space unconstrained by its magnetic field. Filaments are the same thing as prominences, those hot-pink "flames" we see around the edge of the sun during a total solar eclipse. But when they appear against the sun in silhouette, they look like dark ropes or snakes.

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Filaments are made of extremely hot hydrogen called plasma. Plasma looks like a gas, but it's different because its atoms aren't neutral like the ones in the gases we breathe. Heat has disassembled them into individual protons and electrons, an atom's subatomic parts. There's a word for that dismantling — ionization. Plasma is basically an ionized gas, but it's different enough from regular gases to be deemed an entirely different form of matter.

Underlying solar magnetic fields support and suspend filaments (prominences) in the sun's atmosphere similar to the way a hammock suspends its sleepy user above the ground. Every once in a while, the magnetic underpinning of a filament readjusts itself, bolting the blazing rope directly into space in the process. From the filament's point of view, it's a rude awakening. Both the plasma from the coronal hole and two back-to-back filament ejections, traveling at many hundreds of miles an hour, are expected to arrive later today.

NOAA's DSCOVR spacecraft is positioned at the stable sun-Earth Lagrange point (L1), approximately 1 million miles from Earth. From here it monitors our tempestuous sun, providing about an hour's notice of incoming "bad weather." (NOAA)
NOAA's DSCOVR spacecraft is positioned at the stable sun-Earth Lagrange point (L1), approximately 1 million miles from Earth. From here it monitors our tempestuous sun, providing about an hour's notice of incoming "bad weather." (NOAA)

Aurora forecasting is fraught with uncertainty because our knowledge of the powerful solar winds generated by flares and coronal holes is incomplete. Whenever a bundle of plasma heads our way, it carries along a bit of the sun's magnetic field. Until it arrives at the Deep Space Climate Observatory (DSCOVR) located 1 million miles upstream from the Earth, we're in the dark about the particulars. The satellite has sensors that determine the strength and orientation of the arriving cloud and provides up to an hour's notice of storms that can impact our planet.

If the magnetic field bundled with the plasma points north, Earth's magnetic field will deflect it, and we're none the wiser. But if it points south, the solar cloud will open a crack in Earth's magnetic field and eagerly "donate" its plasma into the upper atmosphere. The arriving particles collide at high speed with oxygen and nitrogen atoms at altitudes between 50-400 miles (80-640 km), bumping up their energy levels. When the atoms "relax" back to their previous state, they emit tiny bursts of pink and green light and voilà — the northern lights!

We usually imagine good things happening when things "align." Tonight we hope for the opposite. I'll keep tabs and report back if the aurora shows. You can also check the activity and extent of any auroras at the Aurora 30-Minute Forecast site.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.